State-of-the-art ultrasound imaging systems transmit and receive ultrasound using a transducer that comprises multiple elements forming a transducer array. Imaging is accomplished by transmitting ultrasound pulses and focusing the received echoes to form a two or three-dimensional image.
The transducer array, which typically comprises multiple elements, converts an electrical signal into ultrasound and vice versa. The elements are arranged to form either a one-dimensional (1D) or, increasingly, a two-dimensional (2D) array, and are driven by excitation pulses generated by high-voltage (typically 100 V or greater) transmit circuits. Prior to amplification by high-voltage pulsers, the transmit waveforms are delayed by a transmit beamformer to focus the resulting ultrasound beam at a point of interest. Typically, each element of the transducer array is also amplitude weighted to shape the ultrasound beam by a process known as apodization. The transmit beamformer controls focusing and apodization, along with transmit pulse shape in advanced systems.
Ultrasound waves emitted by the transducer array impact objects in the field that the beam is focused to. Reflected ultrasound waves (echoes) are reflected off of the objects impacted by the emitted ultrasound waves. Echoes that are reflected back to the transducer array are transduced from ultrasonic waves to electrical signals (received signals). The received signals are amplified, filtered, and then, in almost all cases, digitized by analog to digital converters (A/Ds). These digitized signals are then focused and apodized by the receive beamformer, followed by image processing and scan conversion for display on a monitor. Some systems have Doppler circuits to estimate blood flow, whose output is either fed to speakers or displayed on the monitor.
In addition to the provision of circuit blocks to accomplish the above-mentioned functions, ultrasound systems also contain protection circuits in the receive data path. The transducer array elements are inefficient and, as such, require a high-voltage input to generate the ultrasound waves that are emitted by the transducer array. Likewise, due to the inefficiency of the transducer array elements, transduction of the received echoes results in low-voltage received signals. As ultrasound is generated by driving transducers with high-voltage pulses, the low-voltage receive circuits must be isolated from these transmit pulses to prevent permanent damage to the receive circuits. These protection circuits can impose major constraints on ultrasound systems. Size becomes a factor as ultrasound systems shrink into ever-smaller form factors and the number of receive channels (paths) increases. Interconnect complexity, which relates to forming an electrical connection between each transducer element and its receive channel, is complicated by the need to place protection circuit components within this signal path. This interconnect problem is particularly challenging for 2D array-based systems where element and channel counts increase exponentially relative to that of 1D array systems. Component costs can also be significant, as necessary high-voltage components are costly and large. Moreover, active protection circuits (such as high-voltage switches) need power to operate, whereas passive protection circuits (such as diode limiters/expanders) shunt a significant portion of the transmit energy to ground away from the transducer, leading to an inefficient system.
The weak amplitude of the received signal is another major problem, requiring amplification prior to digitization. However, the underlying noise is amplified as well, and front-end components such as the transmit protection circuitry and preamplifier inject additional noise into the signal. This leads to a signal-to-noise ratio (SNR) that can be critically low. As ultrasound waves are attenuated by the medium in which they propagate, low SNR limits the maximum imaging depth.
One strategy to overcome this problem is to reduce the frequency of the transmitted ultrasound pulse, as lower frequencies are attenuated less and return stronger signals to the transducer. However, a lower imaging frequency has the undesirable effect of lowering image resolution.
Another strategy is to simply increase the magnitude of the electrical excitation. A stronger ultrasound wave is then transmitted into the structure being imaged and, consequently, a stronger signal is reflected and received. There are also problems with this approach however as in diagnostic ultrasound imaging, there are strict limitations on the strength of the ultrasound signal that is transmitted into the body. Most of the transmitted ultrasound energy is converted into heat in tissue and there is therefore a danger of overheating tissue. Moreover, too high a magnitude might result in cavitation in tissue, i.e., the production and destruction of microscopic gas bubbles. To eliminate these dangers, the Food and Drug Administration (FDA) places restrictions on the peak negative pressure induced by ultrasound waves in tissue. As peak pressure is directly governed by the magnitude of the transmitted ultrasound signal, ultrasound systems regulate the magnitude of the electrical excitation that drives the transducer.
Coded excitation is a third method that overcomes these challenges to improve the SNR of the received signal. Using coded excitation, the base excitation pulse is convolved with a code that lengthens the transmitted pulse while complying with FDA regulations that govern peak pressure. The resulting longer ultrasound pulse contains more energy, which translates into stronger echoes from targets and a higher SNR in the received signal. The received signal is then “compressed” to eliminate the lengthening effect of the code, while still preserving the extra energy. There are several strategies to compress the received signal: examples include convolution with a matched filter that corresponds to the code or with a custom “mismatched filter” to shape the compressed pulse into one having preferred characteristics. U.S. Pat. No. 6,155,980 entitled “Ultrasonic imaging system with beamforming using unipolar or bipolar coded excitation” assigned to the General Electric Company describes a method for implementing bipolar coded excitation in medical ultrasound systems that have unipolar pulsers and suffer code degradation due to nonlinear propagation. U.S. Pat. No. 6,155,980 is hereby incorporated herein, in its entirety, by reference thereto. A major motivation of the technique was improving SNR. U.S. Pat. No. 6,210,332 entitled, “Method and apparatus for flow imaging using coded excitation” and assigned to the General Electric Company describes the application of coded excitation to improve the SNR of signals used in blood flow estimation. U.S. Pat. No. 6,210,332 is hereby incorporated herein, in its entirety, by reference thereto.
Coded excitation is also used to enhance the speed of ultrasound imaging and increase frame rate. In this scenario, multiple codes are transmitted at the same time or with a short element-to-element delay. This enables simultaneous imaging in different directions and therefore a reduction in imaging time. U.S. Pat. No. 6,213,947 entitled “Medical diagnostic ultrasonic imaging system using coded transmit pulses” assigned to the Acuson Corporation describes one use of coded excitation to increase frame rates. U.S. Pat. No. 6,213,947 is hereby incorporated herein, in its entirety, by reference thereto.
Prior methods for implementing coded excitation, including those cited above, require more hardware resources than are available in imaging systems limited by power, cost and/or area constraints. An example of such a system is the low-cost, hand-held, C-scan ultrasound device described in U.S. patent applications 2006/0052697A1, 2005/0154303A1, 2007/0016044, and 2007/0016022, each of which is hereby incorporated herein, in its entirety, by reference thereto. The C-scan ultrasound device described utilizes a 2D transducer array in conjunction with one unique receive channel for every transducer element. All receive channels acquire the data for a single C-scan image, in parallel. The C-scan images can be formed with as few as four samples (U.S. patent application 2007/0016022), so the integrated circuitry can be fully implemented within a very small area on silicon. Significant power savings are achieved by switching off the receive electronics between relatively sporadic transmit/receive events, which can amount to digitization rates and power duty cycles as slow as the frame rate (˜30 Hz). The small channel area, low digitization rates, and efficient power consumption are directly related to the number of samples that must be acquired and digitized to form each image. Since coded excitation increases the length of the acoustic pulse and since prior methods for implementing coded excitation perform the decoding in the digital domain, the number of samples that must be acquired and digitized to form a C-scan image increases as a multiple of the code length. Such an implementation choice is therefore incompatible with the hardware constraints of the example C-scan ultrasound system and for any system having similar constraints.
There is a continuing need for improving the SNR characteristics of ultrasound systems as the systems become smaller and more complex. The present invention provides solutions for such improvements.